Ground cutting machines of the type referred to commonly operate in extremely hostile environments, and ones in which the cutting points are subjected to enormous mechanical stresses, and, rapid wear by abrasion.
It is, therefore, necessary to form the cutting points as readily replaceable members to permit their replacement at the time the point has become worn down to such an extent that it is no longer efficient as a cutting tool.
The cutting points themselves must of necessity be formed from an extremely hard material that is resistant to wear, and also one which is highly resistant to fatigue fracture in order to provide for an extended life of the points when in use.
The cutting points themselves are thus expensive to manufacture, and, under most circumstances have only a relatively short useful life before the remaining portion of the point must be discarded as scrap material.
For this reason, it has been common practice to support the cutting points on shanks each formed to hold and secure a cutting point. The cutting points are removable and replaceable at the time they have become worn down to an extent that they are of no further use.
One form of shank as disclosed in Smith et al. U.S. Pat. No. 3,750,761, includes a holder into which the cutting point is inserted and then secured in position by means of bolts. Such an arrangement suffers from the disadvantages that the bolts work loose under the sever impacts and vibrations encountered by the tool when in use. Additionally, the heads of the bolts become worn down making them difficult or impossible to remove by a wrench.
Another form of shank disclosed in Gustafson, U.S. Pat. No. 2,222,071, has a planar surface and grooves in its side surfaces that diverge from the planar surface from the open ends of the grooves. The cutting point has a socket of corresponding form, and has side flanges that extend into the grooves of the holder, the side flanges being correspondingly inclined for them to be received within the grooves. In this manner, axial forces which are exerted on the cutting point when in use act to drive the cutting point further into clamping engagement by wedging it further into is associated holder.
However this latter arrangement carries with it the great disadvantage that the extremely expensive cutting point must be manufactured by drop-forging from an expensive material that has compromising characteristics between the extremely high hardness required for the cutting tip of the point and the ductility required for the socket portion. Further, the cutting point must be discarded in its entirety once it has become damaged or worn down, this including the socket portion of the point i.e., a major amount of the expensive material employed in the formation of the cutting point has to be discarded as scrap.
A development of this latter concept is shown in Launder U.S. Pat. No. 4,567,239, in which the relative amount of material to be discarded upon replacement of a cutting points is materially reduced. This is accomplished by forming the point separately from the holding portion of the point.
The holding portion is formed in substantially the same shape as in the prior construction, but, entirely separately from the cutting point. The holding portion is thus in the form of a clamp which is employed for holding the cutting point by wedged onto the supporting shank.
The cutting point is then formed separately with a shank portion of its own formed integrally with the cutting tip, the shank portion being received within the holding clamp and clamped directly onto the shank by the holding clamp.
This construction materially reduces the amount of material that must be discarded on breakage or wearing down of the cutting point, to the cutting point of the tooth and its associated integral shank portion. The holding clamp is not discarded, but instead is available for holding a replacement cutting point, the holding clamp itself being far less subject to wear by abrasion than isthe cutting point itself, and thus lasting for several uses.
Nevertheless, this improvement in the art still goes only part way in meeting the problem, in that the worn cutting point must be discarded in its entirety as scrap, this including the integral shank formed on the cutting point. Clearly, it would be a major advantage if the need to discard a worn cutting point could be eliminated in its entirety. Also, it would be a major advantage if the cutting point could be formed of less expensive material and in a less expensive manner than by drop forging.
Smith et al. U.S. Pat. No. 3750761 teaches a cutting point that can be formed other than by drop forging or casting. Smith teaches the formation of the cutting point of the tooth from a continuous length of bar stock, which can be produced by rolling or drawing, or extrusion. However, Smith's teachings are of attachment of his cutting point to the main shank using the old technology of employing securing bolts, and which is encumbered with all of the disadvantages of the known technology.